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Solstice and Equinox

Most of us understand that Summer and Winter Solstice refer to the longest and shortest days, happening around 21st December and 21st June.  But what is the natural phenomenon that produces these events, along with the Spring and Autumn Equinox?

The shortest day is now behind us and we're on the way back to summer.  As naturists, that will be something that most of us like the thought of!  But, surprisingly, many have not given much thought to how and why this shortest and longest day thing happens at all!  We just accept that in the winter the shadows are long, the temperatures are cooler, and the period of daylight is short.

Astronomy might not be your thing, but the reason behind this phenomenon is really not that difficult to understand.  It basically comes down to one simple fact: the earth is tilted on its axis relative to its journey around the sun.  In fact, it currently leans by about 23.4 degrees.  So, how does this create the seasons for us here in New Zealand?  And how does it make summer days long and winter days short?

Let's just check out this diagram . . .


Here we see our wonderful planet Earth, with its 23.4 degree tilt, on its way around the sun, shown by the red line.  You'll also notice on the right and left positions the Earth has a grey and a white line across it.  The grey line across the center represents the Equator.   The white lines represent the Tropics of Cancer (North of the equator - left position) and Capricorn (South of the Equator - right position).

Let's start at the Summer Solstice on the right hand side.  Some time around December 21 or 22 the earth reaches the point in its orbit around the sun where the tilt of the earth presents the Tropic of Capricorn as the latitude where the sun is directly overhead at midday.  Astronomically speaking, this instant is the moment of mid-summer.  The Tropic of Capricorn lies some 1,500 Km north of Auckland, so it doesn't quite appear directly overhead here, but not far off!  Notice how much of the tropic line is in daylight compared to the amount in darkness.  This is our longest day.  If you look north of the equator the same distance, you'll see our friends in the Northern Hemisphere have a lot more darkness than daylight.  This is the moment of their Winter Solstice; their shortest day.

Following the red line to the top of the diagram, this is where the Earth will be by 20 or 21 March.  This is our Autumnal Equinox.  The Earth now has the Equator as the latitude where the sun is overhead at midday, equi-distant between the North and South Tropics.  This is our Autumn, and the North's Spring.  The length of days and nights are pretty much equal at this point.

Moving on around to the left, the Earth's tilt is now presenting the Northern Tropic, the Tropic of Cancer, closest to the sun.  Around June 20 to 22, places along this Tropic line will have the sun directly overhead at midday.  But for us here in New Zealand, the Tropic of Cancer is 10,900 Km away!  That's why the midday sun appears to us quite low in the northern sky.  Notice also that it's now the Northern Hemisphere's turn to have most of the tropic in daylight compared to the dark hours.  But here in the south, most of our 24 hours are spent in darkness.  This is the moment of our Winter Solstice - the shortest day.

And now we're on our way to Spring.  The Vernal (Spring) Equinox happens around September 22 to 23, when the days and nights once again become more or less equal in length.  The sun is now directly over the Equator, on it's way back to the Tropic of Capricorn once more.

Any questions or comments?  Leave a note in the comments box below!

Roll on Summer!


11 July 2021

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